bug in rendering forceNetwork when arrows = TRUE and Value = NULL

Hi!

I'm trying to create a simple network graph with directed arrows and I only get a strange result back.

Code attached, and resulting graph is shown here:

```
library(networkD3)
source <- c(0,0,1,1)
target <- c(1,2,2,3)
name <- c('zero','one','two','three')
group <- c('Group 1','Group 1','Group 2','Group 2')

links <- data.frame(source,target)
nodes <- data.frame(name,group)

forceNetwork(Links = links, Nodes = nodes,
Source = "source", Target = "target",
NodeID = "name", Group = "group",
linkDistance = 100, zoom = TRUE,
fontSize = 14, opacity = 0.8, legend = T, arrows = T)

```

version 0.4 downloaded using install.packages("networkD3") in RStudio.